LeetCode----- 21.Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

合并两个已经排序的链表，并将其作为一个新链表返回。新的链表应该通过拼接前两个链表的节点来完成。

解题思路：本题比较两个链表中节点的大小，如果当前链表中节点的值小于另外一链表中节点的值，将节点值小的插入到新的链表中。最后需要判断，剩下的链表不为空，需要将其中剩下的节点一个个插入到新的链表中。

代码如下：

public class MergeTwoSortedLists {
    public static ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) {
        	return l2;
        }
        if(l2 == null) {
        	return l1;
        }
        ListNode result = new ListNode(0);
        ListNode pre = l1,rear = l2;
        ListNode currentNode = result;
        ListNode temp = null;
        while(pre != null && rear != null) {
        	if(pre.val < rear.val) {
        		temp = pre;
        		currentNode.next = temp;
        		pre = pre.next;
        	}else {
				temp = rear;
				currentNode.next = temp;
				rear = rear.next;
			}
        	currentNode = currentNode.next;
        }
        while(pre != null) {
        	currentNode.next = pre;
        	pre=pre.next;
        	currentNode = currentNode.next;
        }
        while(rear != null) {
        	currentNode.next = rear;
        	rear=rear.next;
        	currentNode =currentNode.next;
        }
        return result.next;
    }

	public static void main(String[] args) {
		ListNode l10 = new ListNode(1);
		ListNode l11 = new ListNode(3);
		ListNode l12 = new ListNode(5);
		l10.next = l11;
		l11.next = l12;
		l12.next = null;
		
		ListNode l20 = new ListNode(2);
		ListNode l21 = new ListNode(4);
		ListNode l22 = new ListNode(6);
		l20.next = l21;
		l21.next = l22;
		l22.next = null;
		
		ListNode node = mergeTwoLists(l10, l20);
		while(node != null) {
			if(node.next == null) {
				System.out.println(node.val);
			}else{
				System.out.print(node.val +"->");
			}
			node = node.next;
		}
	}
}
class ListNode {
	 int val;
	 ListNode next;
	 ListNode(int x) { val = x; }
}